This invention relates in general to extrusion coating apparatus and, in particular, to an insert usable in an extrusion coating die to modify a distribution channel formed therein.
An extrustion coating apparatus for coating a fluid onto the surface of a web typically comprises at least two members, called "dies". The dies, which are relatively massive dimensionally stable members, are formed of a chemically inert material and are disposed oppositely relative to each other to define an extrusion slot therebetween through which the coating fluid is extruded onto the surface of a web. The dies are typically fabricated of stainless steel although other materials may be used. At least one of the dies has an opening, called the distribution channel, formed therein which extends transversely across a portion thereof. The distribution channel communicates both with the extrusion slot and with a source of coating fluid through a feed passage extending through the body of the die.
In order that the coating fluid exit the mouth of the slot and deposit in a uniform manner across the transverse dimension of the web, the distribution channel must be optimally configured so as to avoid flow discontinuities as the coating fluid is extruded from the slot.
To achieve this optimum configuration the distribution channel must be machined to close tolerances, e.g., on the order of ten-thousandths of an inch. Machining such a relatively massive stainless steel member as an extrusion die to such a high degree of tolerance is expensive. In addition, once a die has been machined to provide the optimum configuration for a coating fluid, the die is not readily adaptable for optimal use with a different fluid since the flow properties of different coating fluids necessitate different cross-sectional configurations of the distribution channel. Therefore, it is either necessary to dedicate an individual die for use in coating a particular fluid onto a web or to machine a distribution channel into a die so that the die may be used to coat more than one fluid. The first expedient is expensive while the second implies a comprise in the configurations of the distribution channels for each of the fluids used.
Accordingly, in view of the foregoing, it is believed to be desirable to provide an arrangement wherein the same coating die may be utilized with a variety of coating fluids and yet the distribution channel in the die is modifiable to permit the die to conform to the optimum configuration for each particular coating fluid used with the die.